Staff work waist-high in muck cleaning doomed pipe

Cleanup workers have so far removed 280 tonnes of material.

Inside the gnarled concrete shell of the sewer pipe across Hobson Bay, four men are wading through a waist-high stream of dark liquid.

As they slog against the swift flow, they pull a heavy flexible hose from a manhole in the sewer top.

It is a giant vacuum cleaner hose, a prized tool for getting down to the nitty-gritty of central Auckland's sewer network.

They push the nozzle under the water and unleash a powerful suction at a bottom layer of grime, grit, gravel and even road metal that has washed into the pipe over decades.

Flying particles ding against the hose wall on their way to a tanker truck stationed on a trestle way over the mudflats at the pipe's Parnell end.

Nearing the end of their physically tough 2.30am to 7.30am shift, the men are thankful that rain held off overnight in the pipeline's central Auckland catchment, or its foul contents would have been chest-high.

Members of the Watercare network service team, based in Mangere, point to a pile of firewood-size chunks of waterlogged timber that have been retrieved from the sewer floor during the shift.

"The other day we found a tree stump in there," said Warwick Brown, Watercare's wastewater operations engineer. "We can't believe the things that get in there."

For five weeks, staff have worked shifts at Hobson Bay, inside the 2.5m-high confines of the pipe.

So far, they have removed 280 tonnes of material, and to complete purging the 3km stretch will take a further four weeks.

However, in a matter of months, there will be no sign of their work. The 98-year-old pipe will be removed.

This will allow more of the tidal bay to be used for water sports.

It will be replaced by a tunnel, which has been bored 25-30m further below the bay and is being prepared for commissioning in December.

Mr Brown said the old sewer pipe's first "spring clean" in 30 years was to solve the demolition problem of contaminated material falling into the Waitemata Harbour.

Materials in the sewer were tainted by harmful heavy metals and hydrocarbons washed down from city streets and yards.

The solution was to pipe it into two specialised trucks owned by contractor HydroTech. Inside the trucks, wastewater is removed and returned to the sewer for processing at the Mangere treatment plant.

DIRTY OLD SECRET

* The 98-year-old sewer pipe across the bay is being replaced by a 3km tunnel.
* But it holds 400 tonnes of contaminated sediment.
* Harmful heavy metals and hydrocarbons have come from city drains.
* Demolishing the old concrete structure risks fouling Waitemata Harbour.
* The job will take nine weeks.
* Demolition will follow a December commissioning of the $118 million tunnel project.